Abstract. A cloud system-resolving model (the Weather Research and
Forecasting model) with 1 km horizontal grid spacing is used to investigate
the response of an idealized supercell storm to increased cloud droplet
concentrations associated with polluted conditions. The primary focus is on
exploring robustness of simulated aerosol effects in the face of complex
process interactions and feedbacks between the cloud microphysics and
dynamics. Simulations are run using sixteen different model configurations
with various microphysical or thermodynamic processes modified or turned
off. Robustness of the storm response to polluted conditions is also
explored for each configuration by performing additional simulations with
small perturbations to the initial conditions. Differences in the
domain-mean accumulated surface precipitation and convective mass flux
between polluted and pristine conditions are small for almost all model
configurations, with relative differences in each quantity generally less
than 15%. Configurations that produce a decrease (increase) in cold pool
strength in polluted conditions also tend to simulate a decrease (increase)
in surface precipitation and convective mass flux. Combined with an analysis
of the dynamical and thermodynamic fields, these results indicate the
importance of interactions between microphysics, cold pool evolution, and
dynamics along outflow boundaries in explaining the system response. Several
model configurations, including the baseline, produce an overall similar
storm response (weakening) in polluted conditions despite having different
microphysical or thermodynamic processes turned off. With hail initiation
turned off or the hail fallspeed-size relation set to that of snow, the
model produces an invigoration instead of weakening of the storm in polluted
conditions. These results highlight the difficulty of foreseeing impacts of
changes to model parameterizations and isolating process interactions that
drive the system response to aerosols. Overall, these findings are robust,
in a qualitative sense, to small perturbations in the initial conditions.
However, there is sensitivity in the magnitude, and in some cases sign, of
the storm response to polluted conditions with small perturbations in the
temperature of the thermal used to initiate convection (less than &pm;0.5 K) or the vertical shear of the environmental wind (&pm;5%). It is
concluded that reducing uncertainty in simulations of aerosol effects on
individual deep convective storms will likely require ensemble methods in
addition to continued improvement of model parameterizations.